changes in both the stratosphere and the troposphere are a source of serious global concern (see Chapter 5).

In reflecting about challenges in modeling the interaction of the physics and the chemistry of the atmosphere it is important to review the extraordinary successes that have been achieved and to recall the scientific and political challenges that faced the planet just a few decades ago.

The Earth's stratosphere contains a thin but crucial layer of ozone that filters out many damaging forms of solar radiation and makes life as we know it possible on the Earth's surface. Beginning in the 1970s, scientists became concerned that certain human-produced chemicals, known as CFCs, could diminish the stratospheric ozone layer.

In the 1980s atmospheric concentrations of CFCs continued to increase, and decreases in stratospheric ozone began to be detected. Stratospheric ozone concentrations over Antarctica plummeted at a remarkable rate during the Antarctic springtime—a phenomenon now referred to as the Antarctic ozone hole. Meanwhile, atmospheric scientists continued working to refine their models of ozone depletion, which had not predicted losses nearly as large as those observed over Antarctica. As a result of a crosscutting scientific effort, including theory, modeling, and observations, a sound scientific basis for the protection of stratospheric ozone was established. In response to the scientific findings, an international agreement was reached to halt production of the most destructive ozone-depleting chemicals. The decisive response of the world community to the stratospheric ozone threat was a tribute to a combined international scientific and policy-making effort.

Advanced three-dimensional atmospheric models were developed to study the interaction of chemistry, dynamics, and radiation in the stratosphere. These extensive calculations were necessary for evaluating the simpler models used in the policy assessment studies as well as for understanding the climatic impact of the Antarctic ozone hole.

Many questions are still unanswered about the future of stratospheric ozone. Will an ozone hole like the one over Antarctica develop over the northern hemisphere in the coming years? How will greenhouse gas-induced climate change interact with stratospheric ozone chemistry? These and other problems, such as the emergence of a global tropospheric ozone problem (see Chapter 5), involving the physics and chemistry of the atmosphere will challenge the scientific community for decades into the future.93

A Modeling Perspective

The goal is a completely interactive simulation of the dynamical, radiative, and chemical processes in the atmosphere. Such a model will be essential in future studies of tropospheric trace constituents such as nitrogen oxides, ozone, and sulfate aerosols. Nitrogen oxides are believed to control the production and

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